Method for manufacturing optical glasses and coloured glasses at low temperatures

ABSTRACT

The invention relates to a method for manufacturing optical glasses and coloured glasses with the aid of a fluid phase sintering process from a basic material encompassing at least SiO 2  powder as well as additives for reducing the temperature of the fluid phase sintering and/or melting process encompassing the following steps: the starting materials are dissolved in any sequence in a fluid medium to produce a solution as far as is possible and a suspension to the extent that they are not dispersed in solution; a greenbody is produced from the dissolved and dispersed starting materials; the greenbody is dried the dried greenbody is fluid-phase sintered at temperatures below 1200° C., in particular in the temperature range from 600° C. to 1200° C.

The invention concerns a method for manufacturing optical glasses and/orcoloured glasses using a sintering or melting process. A sinteringprocess is defined as a process involving a viscosity of 0>1×10⁸ dPa sand a melting process is defined as a process involving a viscosity of0<1×10² dPa s. In particular cases this method can be used tomanufacture multi-component glasses, particularly coloured glass whichis tinted by means of MX semiconductor doping, with M being defined aseither Cd alone or Cd+Zn and X being defined as S, Se or Te.

Essentially two methods for manufacturing glasses by means of asintering process are known by those conversant with current state ofthe art technology:

-   -   the sol-gel process and    -   a method employing powder technology.

One method that can be employed to produce glass by means of a sol-gelprocess is described in U.S. Pat. No. 4,432,956. However, this patentonly describes the production of silica glass manufactured attemperatures of 1300-1500° C.

In order to include further components in glass synthesis one can availoneself of methods such as those described in patents DE 4129409 A1, EP0233732 A1 and U.S. Pat. No. 5,091,115. All these methods are based onsilicon alkoxides such as tetraethyl orthosilicate (TEOS) and othersoluble compounds that as a rule are processed using alcoholic solvents.A base material for SiO₂ can be, for example tetraethyl orthosilicateand the base materials for adding further glass components, such asB₂O₃, Al₂O₃, Na₂O or P₂O₅, can be boric acid trimethyl ester, aluminiumtriisopropylate, sodium methylate, zinc-2,4-pentanedionat, tributylphosphate, or other alkoxides. Some of these raw materials are toxic sothe sol-gel method is disadvantageous in terms of environmental aspectsdue to the raw materials employed and to date have only been used forcoated glasses. A further disadvantage of the sol-gel method is that theprocess is costly due to the expense of the raw materials, requires aconsiderable amount of time and cannot be employed for large componentsdue to the high level of shrinkage in drying and cracking.

A manufacturing process by means of which a CdSSe-doped coloured glasscan be produced using a sol-gel process is described in JP 02221130 A.However, this method employs the same silica alkoxide as its rawmaterial as that described in U.S. Pat. No. 4,432,956, so that it hasthe same disadvantages as all sol-gel methods, particularly in terms ofthe environment.

Another method for producing glass using a sintering process is a methodemploying powder technology. The patent EP 0196140 B1 describes a methodemploying powder technology. The method disclosed in EP 0196140 B1employs a nanoscale Si0₂ powder such as Aerosil OX-50 (DEGUSSA AG) as abase material. In comparison with the sol-gel method this method has theadvantage in the preferred case that no alcoholic raw materials orsolvents have to be used and water can be employed as the dispersionmedium. This results in less environmental contamination, the preventionof unpleasant smelling vapours, the elimination of fire danger and morecost-effective production.

The production of glass in accordance with EP 0196140 B1 is based on thefact that a glass powder suspension is produced and that this suspensionis further processed into a greenbody. The greenbody can be condensed toform transparent, dimensionally stable glass following drying. This hasthe advantage that the intermediate glass product can be produced atroom temperature and retains its shape and form during firing.

However, EP 0196140 B1 only describes the production of silica glassthat is sintered at temperatures between approximately 1300 and 1500° C.These temperatures are too high for employing MX as the doping materialsince a large proportion of the doping material is sublimated oroxidised at these temperatures. Another disadvantage is that the hightemperatures make the process exceptionally energy consuming.

Further methods that employ nanoscale SiO₂ powder and other componentsin addition to SiO₂ are described in the patents JP 62167233 A, JP60171228 A, JP 62100428 A and JP 03159924 A.

However, in accordance with JP 62167233 A, JP 60171228 A, JP 62100428 Aand JP 03159924 A, the nanoscale SiO2 is only utilised as an additionalcomponent to the alkoxide silicates such as for instance TEOS. These rawmaterials and the use of alcoholic solvents also result in environmentalcontamination from the production of these glasses. Moreover, in orderto manufacture glasses in accordance with JP 62167233 A, JP 60171228 A,JP 62100428 A and JP 03159924 A, temperatures have to be employed whichare between 1200 and 2000° C.

U.S. Pat. No. 5,122,178 and JP 60016830 A disclose manufacturingprocesses for CdS, CdSe and CdTe doped glasses. The methods in boththese patents are based, however, on the glass being melted. In U.S.Pat. No. 5,122,178, as in the normal manufacturing process for theseglasses, the doping agent is added during the melting process. The soledifferences with respect to the established manufacturing process arethe modified processing temperatures and process stages. However,relatively high processing temperatures of 1300° C. are still requiredfor melting the glass. The patent JP 60016830 A is based on a two-phaseprocess. Initially a colourless base glass is melted and doped by addingpowdered CdS after the grinding process and then finally sintered. Thedrawbacks to the process described in JP 60016830 A are thetime-consuming and costly multi-stage processing and the fact that it isnot possible to produce the shape of the final product at the greenbodyproduction stage when using the manufacturing method pursuant to JP60016830. This means that the shape of the end product is achieved in JP60016830 A by means of sintering a powder blank. Shaping during thesintering process is particularly difficult, however, when bodies of acomplex shape are to be manufactured.

It is therefore the aim of the invention to make known a productionmethod for optical glasses that overcomes the disadvantages of currentstate-of-the-art technology and in particular, makes it possible tomanufacture coloured glasses using MX doping substances. The method isenvironmentally friendly, cost-effective and energy-efficient andpresents no fire hazard.

In accordance with the invention, this aim is fulfilled by a method inaccordance with Claim 1.

The inventors have recognised the fact that the method in accordancewith the invention is conducted in a viscosity range that correspondsneither with that of the classic sintering process with its viscosity of0>1×10⁵ dPa s nor with that of the classic melting process with itsviscosity of 0<1×10² dPa s, but in a viscosity range between these.Therefore, the process in accordance with the invention is termed fluidphase sintering or high viscosity melting. The advantage over thesintering process is that with the process in accordance with theinvention, the grain boundary between the components and therefore alsothe concentration gradients are reduced not only by the diffusionprocess, but also by plastic flow processes. This results in the processin accordance with the invention being clearly more effective than theclassic sintering process. In order to fully maintain the externalshape, a ceramic aid is used for stabilisation purposes. In comparisonwith a pure melting process, it is the substantially lower temperaturethat constitutes the advantage.

In accordance with the method specified in the invention, a powdery SiO2or SiO₂ suspension encompassing a greenbody and that can be sintered isproduced by means of a method employing powder technology. Furtheradditives are used in addition to the powdery SiO₂ to reduce thetemperature of the sintering process in comparison with that found incurrent state-of-the-art processes and to match the properties of theglass to the doping agent, with no alcoholic solvents being required.

The raw material is SiO₂ powder with a primary particle size between 8nm and 800 nm, preferably 20-100 nm or a SiO₂ suspension. The rawmaterials used as additives are boric acid, zinc oxide, potassiumcarbonate, potassium hydroxide solution and other compounds that havenetwork-modifying properties. However, the additives can also be anyother kind of carbonate, alkaline solutions and bases, such as forinstance a caustic soda solution or potassium hydrogen fluoride. It isirrelevant whether the raw materials are added individually or insuspension or whether two or more of these raw materials are introducedinto the process as previously semi-processed physical or chemicalmulti-component mixing phases. Moreover, the dispersion aids such asammonium fluoride, other alkaline solutions and acids, such as forinstance sulphuric acid or phosphoric acid, can be added. As thesechemicals are also available in standardised, analytically pure form, itis possible to produce highly refined optical glasses with the aid ofthe method in accordance with the invention, with the degree of purityof the glasses being subject to the impurities in the additional glasscomponents.

The production of a glass in accordance with the invention describedhere encompasses the production of a greenbody from the base materialsincluding Si02 powder or SiO₂ suspension and the additives, such as forinstance dispersion aids, and from sintering or melting the glass fromthese greenbodies.

The greenbodies are produced by means of dispersion and dissolution ofthe base materials in any sequence in water, or optionally also in analcoholic solvent.

In order to attain coloured glass, the MX doping substance, such as forinstance CdS, CdSe, CdTe, but also mixed crystals such as CdS/CdSe, isdispersed or dissolved together with the base materials. Instead ofusing CdS, CdSe or CdTe it is also possible to employ in-situ productionof doping substances from the elements, such as Cd+Te→+CdTe, or fromother materials, for instance CdO, Na₂SeO₃ and reduction gases. Otherdyes such as for instance copper indium diselenide (CuInSe²) ortransition metals such as for instance CoO are also possible.

Dissolving and dispersing the base materials and any dopants that may beused for producing a greenbody is undertaken advantageously in such away that this results in a suspension that can be poured or spread. In apreferred embodiment the suspension is poured into a mould. After thesuspension has hardened at room temperature or at temperatures below100° C. the greenbody is dried at room temperature or at temperaturesnot exceeding 400° C. The dried greenbody in then sintered or melted attemperatures between 600-1200° C., preferably between 700-1100° C.,subject to the glass composition.

Optionally, the greenbodies can be ground again or ground again, thendispersed and dried in order to improve the homogeneity and quality ofthe resultant glass.

The glasses can contain common purifiers in their customary quantities.Purifiers are deemed to be those components that release or vaporisegases within the temperature range determined by the method as a resultof redox reactions. Preference is given to purifiers that in addition tohaving a purifying effect have a positive effect upon coloration as aconsequence of the intervention in the redox process. Redox additivesare for instance As₂O₃, Sb₂O₃, AS₂S₃ and Sb₂S₃.

It has proven particularly advantageous for the schliere quality if themaximum proportion of F is 2 wt. %.

The invention is described in an exemplary manner with reference to theembodiments and the FIGURE.

FIG. 1 shows the transmission characteristic of a coloured glassmanufactured in accordance with the invention with 0.234 wt. %CdSe+0.200 CdS (embodiment 3.3) and 0.234 CdSe+0.208 CdS (embodiment3.6).

Firstly, the manufacture of a greenbody for a coloured glass inaccordance with the invention shall be described with reference toembodiments, where CdS is employed as the MX doping substance.

In the embodiments given here the base materials are KOH, H₃BO₃, ZnO,KHF₂ and SiO₂ and the doping substance is US or another dye.

The base materials KOH, H₃BO₃, ZnO, KHF₂, CdS or other dyes and SiO₂ aredissolved or dispersed in the appropriate sequence in water while beingstirred. Optionally, the raw materials can be stirred in with the aid ofultrasound or also by adding additives to the suspension in order tofacilitate the dispersion and dissolution of the various raw materials.

The finished suspension is then poured into a mould in which it hardensand then air-dried at up to 100° C. for 1 to 96 hours. After removalfrom the mould the greenbody is dried for a further 1 to 96 hours atroom temperature and finally for 1 to 48 hours at 40-400° C.

Fluid phase sintering to form transparent glass takes place attemperatures between 600-1200° C., subject to the composition of theglass. The subsequent tempering process for forming CdS crystallites iscarried out at 400-700° C., with soak times being possible that rangefrom 5 to 170 hours.

Glasses can be manufactured in accordance with the invention using thetwo-stage method utilising a fluid phase sintering process attemperatures that are approximately 200 to 700° C. below themanufacturing temperature of glasses when employing a pure meltingmethod. This means that less energy is used during the manufacturingprocess, there is a substantially reduced emission of the toxic dopingmaterials CdS, CdSe and CdTe, there is less chemical corrosion of themelt aggregate as would otherwise result from the aggressive nature ofthe glass composition, and less of the toxic doping material is used. Asa result of it being possible to sinter the greenbody during the courseof the two-phase method in accordance with the invention, it is alsopossible to shape the glass product at room temperature at an earlierpoint in time, substantially reducing the losses associated withprocessing, e.g. during sawing, grinding and polishing. This method isalso extremely environmentally friendly as it is possible to usestandard chemicals found within the chemical and glass-manufacturingindustries for producing greenbodies and the production process iswater-based.

Table 1 shows the final composition of various glasses that have beenproduced using the process in accordance with the invention, as well asthe sintering temperature.

Table 1:

Composition and Sintering Temperature of Coloured Glasses with CdSDoping Manufactured in Accordance with the Invention.

The matching refining temperature is given in Table 2 under thecorresponding example number. The minimal discrepancy in dye contentdoes not result in any noteworthy changes. The total of the base glassis 100%, with the CdS dye being added to this. Example Components 1.11.2 1.3 1.4 1.5 1.6 Composition SiO₂ in % 65.25 56.39 41.23 38.35 40.3441.21 K₂O in % 16.36 16.96 25.19 27.27 23.50 25.43 B₂O₃ in % 7.31 15.1815.39 14.32 11.79 9.81 ZnO in % 10.87 11.28 11.45 10.65 15.78 16.48 F in% 0.21 0.22 6.73 9.40 8.59 7.08 CdS in % 0.73 0.71 0.50 0.49 0.49 0.52Sinter 900° C. 850° C. 700° C. 650° C. 650° C. 700° C. temp.

Table 2 shows the same glasses as in Table 1, with the difference beingthat in Table 2 the doping agent is CdSe. The same glasses have the samenumbers. It is to be noted that the quantity of dopant is dependent uponthe doping agent itself.

In order to improve the quality of the glass, especially with respect tothe minimisation of the number of bubbles, it can be advantageous if theglasses from Table 1 are not only heated to their sintering temperature,but that the temperature is raised slightly higher. This reduces theviscosity of the glasses and any bubbles that may have formed can escapefrom the vitreous body. Table 2 indicates the temperature for eachembodiment at which the bubbles remaining in the glass escape from theglass. This temperature, which is that at which the bubbles escape, istermed the “refining temperature” in the current patent application.

As the temperature values clearly indicate, the “refining temperature”,as defined in this application for the glasses produced in accordancewith the invention, is always below the manufacturing and processingtemperature of glasses manufactured by means of standard methods.

Table 2:

Coloured Glasses with Sharp Cut Characteristic Curves Comparable toThose in Table 1, but Displaced to Higher Wavelengths with the SameComposition, CdSe Doping and Refining Temperature.

The corresponding refining temperature is given in Table 1 under thesame example number. The minimal discrepancy in the dye content does noteffect any noteworthy changes. The total of the base glass is 100%, withthe CdS dye being added to this. Example Components 2.1 2.2 2.3 2.4 2.52.6 Composition SiO₂ in % 65.25 56.39 41.23 38.35 40.21 41.21 K₂O in %16.36 16.96 25.19 27.27 23.50 25.43 B₂O₃ in % 7.31 15.16 15.39 14.3211.79 9.81 ZnO in % 10.87 11.28 11.45 10.65 15.78 16.48 F in % 0.21 0.226.73 9.40 8.59 7.08 CdS in % 0.59 0.57 0.39 0.36 0.38 0.39 Sinter 1200°C. 1150° C. 950° C. 870° C. 890° C. 950° C. temp.

Although the embodiments 1.1 to 2.6 in Tables 1 and 2 are shown asdoped, coloured glasses, the same glasses can also be manufactured witha mixed CdS/CdSe dopant instead of with a CdS or CdSe dopant.

Other colouring dopants are also possible, such as CoO, as is customarywithin the glass industry.

The detailed manufacturing procedure for a greenbody to be sinteredshall now be described for the embodiment 1.6 in an exemplary manner forall other embodiments.

The greenbody for embodiment 1.6 is produced by means of a suspensioncontaining the following substances in 90 litres of water and contains:Original weight Oxide wt. % Raw material (KG) SiO₂ 41.21 SiO₂ 41.21 K₂O25.43 KOH 22.67 B₂O₃ 9.81 H₃BO₃ 17.51 ZnO 16.48 ZnO 16.48 F 7.08 KHF₂14.42 CdS 0.52 CdS 0.52

The various raw materials SiO₂, KOH, H₃BO₃, ZnO, KHF₂, CdS are dissolvedor dispersed in the appropriate sequence in water while being stirred,creating a spreadable suspension that contains all the components of thesubsequent transparent coloured optical glass.

The final suspension is then poured into any mould in which it hardensand then air-dried for 24 hours. After removal from the mould thegreenbody is dried for a further 48 hours at room temperature andfinally for 24 hours at 120° C.

Manufacture by means of fluid phase sintering or high viscosity meltingoccurs at 950° C. and with a soak time of 1 hour. The subsequenttempering process for forming CdS crystallites is carried out at 500°C., with a soak time of 10 hours.

Table 3 which follows gives embodiments of coloured glasses with theaddition of mixed dyes, i.e. CdS/CdSe doping. The total of the basicglass is once again 100%; with the dye CdS/CdSe being added. Thecorresponding refining temperature is indicated in Table 3 under theparticular example number in each case. TABLE 3 Glasses with CdS/CdSedoping Example Components 3.3 3.6 Composition SiO₂ in % 41.23 41.21 K₂Oin % 25.19 25.43 B₂O₃ in % 15.39 9.81 ZnO in % 11.45 16.48 F in % 6.737.08 CdS in % 0.200 0.208 CdSe in % 0.234 0.234 Sinter 980° C. 950° C.temp.

FIG. 1 shows the transmission behaviour of a fluid phase sintered orhighly viscous melted coloured glass with 0.234 CdSe+0.200 CdS inaccordance with embodiment 3.3 and embodiment 3.6 in Table 3 in awavelength range from 500-700 mm. In Table 3 embodiment 3.3 has thereference number 1, while embodiment 3.6 has the reference number 2 inTable 3.

The method in accordance with the invention makes known for the firsttime a method which when compared to conventional manufacturingprocesses, whereby these glasses are melted from various raw materials,initially involves the production of a greenbody that is compressed toform transparent glass. This manufacturing process dispenses with theneed to heat the glass raw materials to their melting temperature in theviscosity range <10² dPa. Consequently, the process temperatures can bereduced significantly. In comparison with conventional methods thatrequire 1200-1400° C. to melt the glass, this method in accordance withthe invention only utilises a temperature ranging from 600-1200° C. Thisis particularly interesting in the manufacture of coloured glasses whichcan be employed, for instance as sharp cut filters, as the MX dopantrequired for coloured glasses is toxic and highly volatile at hightemperatures. Therefore, in comparison with conventional melting methodsfor manufacturing glasses of this type, in addition to using far lessenergy the method in accordance with the invention also has theadvantage that the lower temperature and shorter resting period meansthat the melting and/or sintering temperature and the emission of thedopant is significantly reduced. It is also for this reason that themethod in accordance with the invention is also superior to existingmethods in terms of environmental aspects.

Moreover, the extremely aggressive glass melt is prevented fromcorroding the melt aggregate excessively, which would otherwise make theunit rapidly inoperable.

1-19. (canceled)
 20. A glass manufactured in accordance with thefollowing method: providing starting materials comprising SiO₂ andoptionally, additives for reducing a sintering or melting temperature ofthe glass, the starting materials not including silicon alkoxides;dissolving the starting materials in any sequence in a fluid medium toproduce a solution as far as is possible and a suspension to the extentthat the starting materials are dispersed in solution; producing agreenbody from the dissolved and dispersed starting materials; dryingthe greenbody; and fluid-phase sintering the dried greenbody attemperatures below 1200° C., in particular in the temperature range from600° C. to 1200° C.; wherein the glass comprises: 30-75 wt. % SiO₂ 5-35wt. % K₂O 4-17 wt. % B₂O₃ 0-37 wt. % ZnO 0.01-10 wt. % F
 21. A glass inaccordance with claim 20 wherein the glass includes F in the range0.01-2 wt. %.
 22. A glass in accordance with claim 20 wherein the glassalso or alternatively includes: the oxides of the elements Al, Ti, Zr,P, Mg, Ca, Sr, Ba, Ce, La, Na, Li, Rb, Cs, Pb.
 23. A glass manufacturedin accordance with the following method: providing starting materialscomprising SiO₂ and optionally, additives for reducing a sintering ormelting temperature of the glass, the starting materials not includingsilicon alkoxides; dissolving the starting materials in any sequence ina fluid medium to produce a solution as far as is possible and asuspension to the extent that the starting materials are dispersed insolution; producing a greenbody from the dissolved and dispersedstarting materials; drying the greenbody; and fluid-phase sintering thedried greenbody at temperatures below 1200° C., in particular in thetemperature range from 600° C. to 1200° C.; wherein the glass comprises:30-75 wt. % SiO₂ 5-35 wt. % K₂O 4-17 wt. % B₂O₃ 0-30 wt. % ZnO 0.01-10wt. % F
 24. A glass in accordance with claim 23 wherein the glass alsoor alternatively includes: the oxides of the elements Al, Ti, Zr, P, Mg,Ca, Sr, Ba, Ce, La, Na, Li, Rb, Cs.
 25. A glass in accordance with claim24, including: 30-75 wt. % SiO₂ 5-35 wt. % K₂O 4-17 wt. % B₂O₃ 0-30 wt.% ZnO 0.01-10 wt. % F 0-5 wt. % TiO₂
 26. A glass in accordance withclaim 23 wherein the glass is a coloured glass which include an MXsemiconductor dopant with M Cd and/or Zn and X S, Se or Te.
 27. Acolored glass in accordance with claim 26 wherein as an option the glassis doped with ionic dyes such as Co, Cu, Ni etc. instead of or inaddition to the MX semiconductor dopants.
 28. A colored glass inaccordance with claim 26 wherein the MX semiconductor dopant has 0.2-0.9wt. % of the optical glass.
 29. An optical sharp cut filter forwavelengths between 350 nm and 850 nm wherein the optical sharp cutfilter includes a coloured glass in accordance with claim 26.